Re: IOMMU Query

["Roedel, Joerg" <Joerg.Roedel@amd.com>]

On Wed, Mar 23, 2011 at 05:08:45AM -0400, Abhilash Kesavan wrote:
> We are investigating if the generic IOMMU API (include/linux/iommu.h)
> can be used for the IOMMU on our arm-based boards.

Great :-)

> I noticed that msm's IOMMU code (arch/arm/mach-msm/iommu.c) uses the
> genric IOMMU API and have a few queries regarding the supported
> features. Following are some features/requirements of our system:
> 
> 1) Multiple IOMMU's  - consistency between parallel IOMMUs must be maintained

This is hidden behind the API. The IOMMU driver is responsible for
managing multiple IOMMUs. On x86 there is a unique device<->IOMMU
relationship which is used internally in the drivers. Users of the
IOMMU-API can ignore the fact that there are multiple IOMMUs in the
system.

> 2) Supports varying page sizes - 4K, 64K, 1M, 16M i.e. a sg list of
> contiguous physical memory blocks be made available. Thus, giving us
> mappings from the conservative 4K to large1M reducing TLB misses.

This is supported but also hidden in the IOMMU driver implementation.
the AMD-IOMMU driver implements multiple page-sizes already using this
API. The API user has a function which looks as follows:

	int iommu_map(struct iommu_domain *domain,
		      unsigned long iova,
		      phys_addr_t paddr,
		      int gfp_order,
		      int prot);

this function needs to map the pysically contiguous memory starting at
paddr at the io-virtual address iova. The size of the mapping is
determinded by the gfp_order parameter. The size of the mapping is
basically (2**(gfp_order))*PAGE_SIZE. Note that PAGE_SIZE is the cpu
page-size.
The IOMMU driver has enough information from that function to map the
area with the best-fit page-sizes.

> 3) Multiple VA allocations can be associated with the same PA space.

I don't really understand what you mean here. Can you elabortate?

> Who manages the virtual address space of IO device that has IOMMU? I
> think the generic iommu.h API cannot handle virtual address space and
> memory allocation.  Is it just a IOMMU handler? Should we have another
> virtual memory manager for IOMMU?

The layout of the virtual address space for the IO device needs to be
managed by the particular device driver. The IOMMU-API only provides
functions for mapping/unmapping memory.

Regards,

	Joerg

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